Genetic, biochemical and biophysical analyses are being employed in order to describe the mechanism whereby small molecular weight compounds modulate the activity of the regulatory enzyme, L-threonine deaminase from Salmonella typhimurium. In vitro transcription systems are being developed in order to define the mechanism whereby the genes which specify the isoleucine-valine biosynthetic enzymes in Salmonella typhimurium are multivalently regulated by the branched chain amino acids.